Device and method for binary flag determination

ABSTRACT

An embodiment method for determining a carry digit indicator bit of a first binary datum includes a step for processing of the first binary datum masked by a masking operation, and not including any processing step of the first binary datum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of French Provisional Application No. 1911347, filed on Oct. 11, 2019, which application is hereby incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to electronic systems and circuits, and more specifically to processors and associated methods. The present disclosure more specifically relates to processors configured to process masked data, and methods of operating thereof.

BACKGROUND

A processor is an electronic component, present in many electronic systems and circuits, that is configured to process data by executing commands and instructions from computer programs.

In some cases, a processor may have to process secret data. These secret data are generally encrypted, for example by masking.

It would be desirable to be able to improve, at least partially, certain aspects of known processors.

SUMMARY

There is a need in the art for more reliable processors.

There is a need in the art for processors configured to process masked data.

There is a need in the art for processors configured to process masked data without implementing unmasking operations of these data.

One embodiment addresses all or some of the drawbacks of known processors.

One embodiment provides a method for determining a carry digit indicator bit of a first binary datum including a step for processing of the binary datum masked by a masking operation, and not including any processing step of the first datum.

According to one embodiment, the first datum is not unmasked during the determination.

According to one embodiment, the first datum is obtained by performing the addition of a second masked datum and a third masked datum.

According to one embodiment, the masked datum is masked by adding a mask to the datum to be masked.

According to one embodiment, the carry digit indicator bit is provided by the following equivalent formulas:

Flag_C=CD1_M xor CD2_M xor CADD_D_M xor CADD_M xor CD_MD

Flag_C=CD1_M+CD2_M+CADD_D_M−CADD_M−CD_M

wherein:

-   -   xor represents the EXCLUSIVE OR logic operation;     -   “+” represents the addition operation;     -   − represents the subtraction operation;     -   CD1_M represents the carry digit that may appear during the         masking operation leading to the second masked datum;     -   CD2_M represents the carry digit that may appear during the         masking operation leading to the third masked datum;     -   CADD_D_M represents the carry digit that may appear during the         addition operation between the second and third masked data;     -   CADD_MD represents the carry digit that may appear during the         addition operation between the second and third masks; and     -   CD_M the carry digit that may appear during the masking         operation leading to the first masked datum.

Another embodiment provides a circuit configured to carry out the method previously described.

According to one embodiment, the circuit comprises three comparators.

According to one embodiment, the circuit further comprises one circuit implementing a logic gate of the EXCLUSIVE OR type.

According to one embodiment, the circuit implementing a logic gate of the EXCLUSIVE OR type comprises five inputs.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:

FIG. 1 shows, schematically and in block diagram form, part of an architecture of a processor;

FIG. 2 shows, schematically and in block diagram form, an embodiment of a circuit of the processor of FIG. 1;

FIG. 3 shows, schematically and in block diagram form, an embodiment of another circuit of the processor of FIG. 1; and

FIG. 4 shows, schematically and in block diagram form, a detailed example of an embodiment of a processor of FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties.

For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. The complete operation of a processor will not be disclosed here, the embodiments disclosed below being compatible with most of the typical operating methods of a processor.

Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements.

In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures.

Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%.

In the remainder of the disclosure, the data are binary data, that is to say, each datum corresponds to a binary word comprising at least one bit, preferably more than one bit.

FIG. 1 shows, schematically and in block diagram form, part of an architecture of one embodiment of a processor 10 (CPU).

The processor 10 is a processor configured to process masked data and their masks. The processor 10 comprises an arithmetic and logic unit 100 (ALU) that is also configured to process the masked data and their masks. More specifically, the processor 100 receives the masked input data Data_In and their masks Mask_In, as input, and provides masked output data Data_Out, and their masks Mask_Out, as output.

The input data Data_In, respectively the output data Data_Out, are masked with the masks Mask_In, respectively Mask_Out, by implementing masking of the arithmetic type. Arithmetic masking is, in the case described here, additive masking in which the mask is added to the datum to be masked. The mask and the datum to be masked are binary words of equal size. More specifically, a masked datum A_M is given by the following formula:

A_M=(A+MA)mod 2^(n)

wherein:

-   -   A represents the datum to be masked;     -   MA represents the mask;     -   “+” represents the addition operation;     -   “mod” represents the modulo operation; and     -   n is the number of bits that makes up the datum to be masked A,         the mask MA and the masked datum A_M.

The unit wo is configured to further receive an operating code Opcode explaining the processing to be applied to pairs each comprising a masked input datum Data_In and its mask Mask_In. The operating code Opcode indicates the or the set of different operations to be applied to the masked input data Data_In, and their masks Mask_In, in order to obtain the masked output data Data_Out and their masks Mask_Out. An operating code is more specifically a set of instructions defining the or the set of arithmetic and/or logic operations to be applied to the input data Data_In and their masks Mask_In.

In order to implement these operating codes, the unit wo comprises one or several, preferably several, circuits 101 (OP) configured to carry out various arithmetic or logic operations. The circuits 101 are configured to be selected and used as a function of the set of instructions defined by the operating code Opcode received by the unit 100. As an example, the circuits 101 are configured to carry out arithmetic operations such as an addition, a multiplication, a complementary calculation, etc., and/or logic operations such as the AND operation, the OR operation, the EXCLUSIVE OR (XOR) operation, etc. Several circuits 101 can be implemented in parallel or in series in order to execute the operating code Opcode. Each circuit 101 is thus configured to receive masked input data Data_In, and their masks Mask_In, and/or intermediate masked output data Data_Int, and their masks Mask_Int, corresponding to output data, and their masks, of other circuits 101.

The unit 100 further comprises circuits 103 (FL) for generating indicator bits, or flag bits. Each circuit 103 is coupled to one of the circuits 101. The circuit 103 is configured to receive, as input, the masked data received, as input, by the circuits 101, and their masks, and the data supplied, as output, by the circuits 101, and their masks. Thus, in the example disclosed in relation with FIG. 1, the circuit 103 is configured to receive masked data Data_In, and their masks Mask_In (not shown in FIG. 1), and intermediate output data Data_Int, and their masks Mask_Int. The circuit 103 supplies indicator bits as output. More specifically, the circuits 103 are configured to supply:

-   -   a sign indicator bit Flag_N;     -   a carry digit indicator bit Flag_C;     -   an indicator bit of a null value Flag_Z; and     -   an overflow indicator bit Flag_V.

According to one variant, certain parts of the circuits 103 of the unit 100 can be shared by several circuits 101.

The sign indicator bit Flag_N is a bit equal to “1” when the datum from which it is calculated is a negative number, and equal to “0” otherwise. One exemplary circuit configured to calculate the indicator bit Flag_N is disclosed in relation with FIG. 2.

The carry digit indicator bit Flag_C is a bit equal to “1” when the datum from which it is calculated comprises a carry digit, and equal to “0” otherwise. One exemplary circuit configured to calculate the indicator bit Flag_N is disclosed in relation with FIG. 3.

The indicator bit of a null value Flag_Z is a bit equal to “1” when the datum from which it is calculated is equal to zero, and equal to “0” otherwise.

The overflow indicator bit Flag_V is a bit equal to “1” when the datum from which it is calculated is a number too large to be represented by the number of bits of the datum from which it was calculated, and equal to “0” otherwise.

According to one embodiment, the indicator bits Flag_N, Flag_C, Flag_Z and Flag_V are generated each time the circuit 101 is used. According to one variant, only some of the indicator bits Flag_N, Flag_C, Flag_Z and Flag_V are generated for several uses of the circuit 101 or for another type of circuit 101.

FIG. 2 shows, schematically and in block diagram form, an embodiment of a circuit 20 (Flag_N) configured to calculate the sign indicator bit Flag_N disclosed in relation with FIG. 1.

The circuit 20 receives, as input, a masked datum B_M and its mask MB, the datum B_M being obtained from a datum B. The datum B_M and the mask MB are binary words comprising n bits, n being a natural integer. The following notation will be used hereinafter:

-   -   P[m; k] designates the set of bits going from rank k to rank m         of a binary word P, m and k being natural integers less than or         equal to n; and     -   P[m] designates the bit with rank m of the binary word P.

As disclosed in relation with FIG. 1, the sign indicator bit Flag_N is a bit equal to “1” when the datum from which it is calculated, that is to say the datum B here, represents a negative number, and is equal to “0” otherwise. By convention, the most significant bit of a binary word representing a negative number is equal to “1”. According to one embodiment, the masked datum B_M is negative when the following condition is met:

(B_M[n−1;0]+CB_M*2^(n))≥(MB[n−1;0]+2^(n−1))

wherein CB_M represents the carry digit that may appear during the masking operation leading to the masked datum B_M.

The carry digit CB_M is equal to “1” when the following condition is met, otherwise it is equal to “0”:

B_M[n−1;0]≤MB[n−1;0]

One advantage of the circuit disclosed in relation with FIG. 2 is that it makes it possible to provide the sign indicator bit Flag_N of a masked datum without having to unmask the datum.

FIG. 3 shows, schematically and in block diagram form, an embodiment of a circuit 30 (Flag_C) configured to calculate the carry digit indicator bit Flag_C disclosed in relation with FIG. 1.

The circuit 30 is a circuit more specifically belonging to a circuit 103, disclosed in relation with FIG. 1, associated with a circuit 101 carrying out an addition operation between two masked data. More specifically, an output datum D_M and its mask MD of the circuit 101 are given by the following formulas:

$\left\{ {\begin{matrix} {{D\_ M} = {\left( {{D\; 1{\_ M}} + {D\; 2{\_ M}}} \right){mod}\; 2^{n}}} \\ {{MD} = {\left( {{{MD}\; 1} + {{MD}\; 2}} \right){mod}\; 2^{n}}} \end{matrix}\quad} \right.$

wherein:

-   -   D1_M and D2_M represent two data D1 and D2 after a masking         operation; and     -   MD1 and MD2 represent the masks of the masked data D1_M and         D2_M.

As disclosed in relation with FIG. 1, the carry digit indicator bit Flag_C is a bit equal to “1” when the datum from which it is calculated comprises a carry digit, and equal to “0” otherwise. The carry digit indicator bit Flag_C is more specifically given by the following equivalent formulas:

Flag_C=CD1_M xor CD2_M xor CADD_D_M xor CADD_MD xor CD_M

Flag_C=CD1_M+CD2_M+CADD_D_M−CADD_MD−CD_M

wherein:

-   -   xor represents the EXCLUSIVE OR logic operation;     -   CD1_M represents the carry digit that may appear during the         additive masking operation leading to the masked datum D1_M with         the mask MD1;     -   CD2_M represents the carry digit that may appear during the         additive masking operation leading to the masked datum D2_M with         the mask MD2;     -   CADD_D_M represents the carry digit that may appear during the         addition operation between the two masked data D1_M and D2_M;     -   CADD_MD represents the carry digit that may appear during the         addition operation between the two masks MD1 and MD2; and     -   CD_M represents the carry digit that may appear during the         additive masking operation leading to the masked datum D_M with         the mask MD.

The carry digits CD1_M, CD2_M and CD_M are obtained by comparing, with its mask, the masked datum with which they are each associated, in the same manner that the carry digit CB_M is obtained as disclosed in relation with FIG. 2.

The carry digit CADD_D_M is equal to “1” when the following condition is met:

D_M[n−1;0]<(D1_M[n−1;0]+D2_M[n−1;0])mod 2^(n)

The carry digit CADD_D_M is equal to “0” otherwise. The carry digit CADD_D_M is also the bit of rank n resulting from the addition of the binary words D1_M[n−1;0] and D2_M[n−1;0].

The carry digit CADD_MD is equal to “1” when the following condition is met:

MD[n−1;0]<(MD1[n−1;0]+MD2[n−1;0])mod 2^(n)

The carry digit CADD_MD is equal to “0” otherwise. The carry digit CADD_MD is also the bit of rank n resulting from the addition of the binary words MD1[n−1;0] and MD2[n−1;0].

One advantage of the circuit 30 disclosed in relation with FIG. 3 is that it does not use any unmasking operation of the masked data D_M, D1_M and D2_M to calculate the carry digit indicator bit Flag_C.

FIG. 4 shows, schematically and in block diagram form, a more detailed example of an embodiment of a circuit 103 associated with a circuit 101.

The circuit 101 is a circuit configured to carry out an addition or subtraction operation between two masked data E_M and F_M, and their masks ME and MF, to provide, as output, a masked datum G_M and its mask MG. The masked data E_M, respectively F_M, G_M, are the result of an additive masking of a datum E, respectively F, G, with the mask ME, respectively MF, MG. In the case of addition, the masked datum G_M and its mask MG are given by the following formulas (A):

$\begin{matrix} \left\{ \begin{matrix} {{G\_ M} = {\left( {{E\_ M} + {F\_ M}} \right){mod}\; 2^{n}}} \\ {{MG} = {\left( {{ME} + {MF}} \right){mod}\; 2^{n}}} \end{matrix} \right. & (A) \end{matrix}$

In the case of subtraction, the masked datum G_M and its mask MG are given by the following formulas (B):

$\begin{matrix} \left\{ \begin{matrix} {{G\_ M} = {{\left( {{E\_ M} - {F\_ M}} \right){mod}\; 2^{n}} = {\left( {{E\_ M} + {!{{F\_ M} + 1}}} \right){mod}\; 2^{n}}}} \\ {{MG} = {{\left( {{ME} + {MF}} \right){mod}\; 2^{n}} = {\left( {{ME} + {!{{MF} + 1}}} \right){mod}\; 2^{n}}}} \end{matrix} \right. & (B) \end{matrix}$

wherein !X designates the complementary to 1 of a binary word X.

The circuit 101 comprises two adder circuits 1011 and 1012, and two inverter circuits 1013 and 1014. As an example, the inverter circuits 1013 and 1014 are EXCLUSIVE OR logic circuits used as inverters in the case of a subtraction operation, or as followers in the case of an addition operation.

The bitwise addition circuit 1011 is a circuit comprising at least three inputs and at least two outputs. The circuit 1011 receives, as input, the mask ME, a mask MF′ modified from the mask M, and operation information ADD_SUB. As an example, the operation information ADD_SUB is equal to “0” in the case of addition and “1” in the case of subtraction. The circuit 1011 provides, as output, the mask MG and a carry digit C1011.

The addition circuit 1012 is a circuit comprising at least three inputs and at least two outputs. The circuit 1012 receives, as input, the masked datum E_M, a masked datum F_M′ modified from the masked datum F_M, and the operation information ADD_SUB. The circuit 1012 provides, as output, the mask MG and a carry digit C1012.

The inverter circuit 1013 makes it possible to modify the mask MF to the mask MF′ as a function of the operation information ADD_SUB.

The inverter circuit 1014 makes it possible to modify the masked datum F_M to the masked datum F_M′ as a function of the operation information ADD_SUB.

The operation of the circuit 101 is as follows. The operation information ADD_SUB is equal to “0” when an addition operation is requested, and equal to “1” when a subtraction operation is requested.

When an addition operation is requested, the inverter circuits 1013 and 1014 see operation information ADD_SUB equal to “0” that indicates to them that they are not asked to reverse the datum that they receive as input. The circuits 1013 and 1014 then supply, as output, the mask MF′ equal to the mask MF, and the masked datum F_M′ equal to the masked datum F_M. The adder circuits 1011 and 1012 and the data that they receive as input, and then supply, as output, the mask MG and the masked datum G_M according to the formulas (A) given above.

When a subtraction operation is requested, the inverter circuits 1013 and 1014 see operation information ADD_SUB equal to “1” that indicates to them that the calculation of the complementary of the datum that they receive as input is requested. The circuits 1013 and 1014 then supply, as output, the mask MF′ equal to the mask MF, and the masked datum F_M′ equal to the masked datum F_M. The adder circuits 1011 and 1012 and the data that they receive as input, and then supply, as output, the mask MG and the masked datum G_M according to the formulas (B) given above.

The circuit 103 is adapted to supply the indicator bits Flag_V, Flag_C, Flag_N and Flag_Z disclosed in relation with FIG. 1. The circuit 103 receives, as input:

-   -   the mask ME;     -   the masked datum E_M;     -   the mask MF;     -   the masked datum F_M;     -   the operation information ADD_SUB;     -   the carry digit C1011 coming from the adder circuit 1011;     -   the carry digit C1012 coming from the adder circuit 1012;     -   the mask MG; and     -   the masked datum G_M.

The circuit 103 comprises:

-   -   three comparator circuits 1031 (COMP1), 1032 (COMP2) and 1033         (COMP3);     -   two circuits 1034 and 1035 implementing a logic gate of the AND         type;     -   one circuit 1036 implementing a logic gate of the OR type; and     -   one circuit 1037 implementing a logic gate of the EXCLUSIVE OR         type.

The comparator circuit 1031 receives, as input, the mask ME and the masked datum E_M, and supplies, as output, a binary word CE_M representing the carry digit that may appear during the masking operation leading to the masked datum E_M. Additionally, the comparator circuit 1031 supplies, as output, a sign indicator bit FlagE_N indicating the sign of the datum E corresponding to the masked datum E_M unmasked with the mask ME.

The comparator circuit 1032 receives, as input, the mask MF, the masked datum F_M, and the operation information ADD_SUB, and supplies, as output, a binary word CF_M representing the carryover digit that may appear during the masking operation leading to the masked datum F_M in the case of an addition, and representing the inverse of the carryover digit that may appear during the masking operation leading to the masked datum F_M in the case of a subtraction. Additionally, the comparator circuit 1032 supplies, as output, a sign indicator bit FlagF_N indicating the sign of the datum F corresponding to the masked datum F_M unmasked with the mask MF in the case of an addition, or the inverse of the sign of F in the case of a subtraction.

The comparator circuit 1033 receives, as input, the mask MG and the masked datum G_M, and supplies, as output, a binary word CG_M representing the carry digit that may appear during the masking operation leading to the masked datum G_M. Additionally, the comparator circuit 1033 supplies, as output, a sign indicator bit FlagG_N indicating the sign of the datum G corresponding to the masked datum G_M unmasked by [sic] with the mask MG, and an indicator bit of a null value FlagG_Z indicating whether the datum G is equal to zero. The sign indicator bit Flag_N is equal to the indicator bit FlagG_N. The indicator bit of a null value Flag_Z is equal to the indicator bit FlagG_Z.

The circuit 1034 is a circuit implementing a logic gate of the AND type comprising one non-inverter input and two inverter inputs. The circuit 1034 receives, as input, on its non-inverter input, the sign indicator bit FlagG_N, and on its two inverter inputs, the sign indicator bit FlagE_N and the sign indicator bit FlagG_N. The circuit 1034 provides, as output, a datum Flag_V4.

The circuit 1035 is a circuit implementing a logic gate of the AND type comprising two non-inverter inputs and one inverter input. The circuit 1034 receives, as input, on its non-inverter inputs, the sign indicator bit FlagE_N and the sign indicator bit FlagF_N, and on its inverter input, the sign indicator bit FlagG_N. The circuit 1035 provides, as output, a datum Flag_V5.

The circuit 1036 is a circuit implementing an OR logic gate comprising two inputs. The circuit 1036 receives the outputs of the circuits 1035 and 1036 as input, that is to say, the data Flag_V4 and FlagV5. The circuit 1036 provides, as output, the overflow indicator bit Flag_V.

The circuit 1037 is a circuit implementing a logic gate of the EXCLUSIVE OR type comprising five non-inverter inputs. The circuit 1037 receives, as input, the carry digit CE_M, the carry digit CG_M, the carry digit C1011 and the carry digit C1012. The circuit 1037 provides, as output, the carry digit indicator bit Flag_C.

One advantage of the circuit 103 disclosed in relation with FIG. 4 is that it does not use unmasking operations to calculate the indicator bits Flag_V, Flag_C, Flag_N and Flag_Z.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art.

Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional description provided hereinabove. 

What is claimed is:
 1. A method comprising: masking, by a masking operation, a first binary datum; processing the masked first binary datum; and determining a carry digit indicator bit of the first binary datum without directly processing the first binary datum.
 2. The method according to claim 1, further comprising keeping the masked first binary datum masked during the determining the carry digit indicator bit.
 3. The method according to claim 1, wherein the masking the first binary datum comprises adding a mask to the first binary datum.
 4. The method according to claim 1, further comprising obtaining the masked first binary datum by performing an addition of a second masked datum and a third masked datum.
 5. The method according to claim 4, wherein the carry digit indicator bit is determined by one of the following formulas: Flag_(c)=CD1_M xor CD2_M xor CADD_D_M xor CADD_MD xor CD_M; or Flag_C=CD1_M+CD2_M+CADD_D_M−CADD_MD−CD_M where: Flag_C represents the carry digit indicator bit; “xor” represents an EXCLUSIVE OR logic operation; “+” represents an addition operation; “−” represents a subtraction operation; CD1_M represents a first carry digit that may appear during the masking operation leading to the second masked datum; CD2_M represents a second carry digit that may appear during the masking operation leading to the third masked datum; CADD_D_M represents a third carry digit that may appear during the addition of the second masked datum and the third masked datum; CADD_MD represents a fourth carry digit that may appear during an addition of a second mask and a third mask; and CD_M represents a fifth carry digit that may appear during the masking operation leading to the masked first binary datum.
 6. The method according to claim 1, further comprising setting the carry digit indicator bit to “1” to indicate a presence of a carry digit.
 7. The method according to claim 1, further comprising receiving an operating code indicating one or more operations to be performed by the processing.
 8. The method according to claim 1, further comprising generating an output binary datum and an associated masked datum in accordance with the processed masked first binary datum and the determined carry digit indicator bit.
 9. A circuit configured to: mask a first binary datum; process the masked first binary datum; and determine a carry digit indicator bit of the first binary datum without directly processing the first binary datum.
 10. The circuit according to claim 9, further comprising three comparators configured to determine the carry digit indicator bit.
 11. The circuit according to claim 9, further comprising an EXCLUSIVE OR logic gate configured to process the masked first binary datum.
 12. The circuit according to claim 11, wherein the EXCLUSIVE OR logic gate comprises five inputs configured to receive two masked data, their associated masks, and an addition/subtraction selection.
 13. The circuit according to claim 9, further configured to keep the masked first binary datum masked when determining the carry digit indicator bit.
 14. The circuit according to claim 9, wherein the circuit configured to mask the first binary datum comprises the circuit configured to add a mask to the first binary datum.
 15. The circuit according to claim 9, further configured to obtain the masked first binary datum by performing an addition of a second masked datum and a third masked datum.
 16. The circuit according to claim 15, wherein the circuit is configured to determine the carry digit indicator bit by one of the following formulas: Flag_(c)=CD1_M xor CD2_M xor CADD_D_M xor CADD_MD xor CD_M; or Flag_C=CD1_M+CD2_M+CADD_D_M−CADD_MD−CD_M where: Flag_C represents the carry digit indicator bit; “xor” represents an EXCLUSIVE OR logic operation; “+” represents an addition operation; “−” represents a subtraction operation; CD1_M represents a first carry digit that may appear during the masking operation leading to the second masked datum; CD2_M represents a second carry digit that may appear during the masking operation leading to the third masked datum; CADD_D_M represents a third carry digit that may appear during the addition of the second masked datum and the third masked datum; CADD_MD represents a fourth carry digit that may appear during an addition of a second mask and a third mask; and CD_M represents a fifth carry digit that may appear during the masking operation leading to the masked first binary datum.
 17. The circuit according to claim 9, further configured to set the carry digit indicator bit to “1” to indicate a presence of a carry digit, or to “0” to indicate an absence of the carry digit.
 18. The circuit according to claim 9, further configured to receive an operating code indicating one or more operations to be performed on the masked first binary datum.
 19. The circuit according to claim 9, further configured to generate an output binary datum and an associated masked datum in accordance with the processed masked first binary datum and the determined carry digit indicator bit.
 20. A circuit comprising: five inputs configured to receive two masked data, their associated masks, and an addition/subtraction selection bit; an EXCLUSIVE OR (XOR) logic gate coupled to the five inputs, and configured to add or subtract the two masked data and their associated masks in accordance with the addition/subtraction selection bit, to generate a masked datum and its associated mask; and three comparators coupled to the five inputs and the XOR logic gate, and configured to generate a carry digit indicator bit in accordance with the five inputs, the masked datum and its associated mask. 